Textbook Question
The acid–base indicator bromcresol green is a weak acid. The yellow acid and blue base forms of the indicator are present in equal concentrations in a solution when the pH is 4.68. What is the pKa for bromcresol green?
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Ch.17 - Additional Aspects of Aqueous Equilibria
Chapter 17, Problem 89
A biochemist needs 750 mL of an acetic acid–sodium acetate buffer with pH 4.50. Solid sodium acetate (CH3COONa) and glacial acetic acid (CH3COOH) are available. Glacial acetic acid is 99% CH3COOH by mass and has a density of 1.05 g/mL. If the buffer is to be 0.15 M in CH3COOH, how many grams of CH3COONa and how many milliliters of glacial acetic acid must be used?
Verified step by step guidance1
Calculate the moles of CH3COOH needed using the formula: moles = molarity \times volume. Use the given molarity (0.15 M) and convert the volume from mL to L (750 mL = 0.750 L).
Determine the mass of CH3COOH required using the moles calculated and the molar mass of CH3COOH (60.05 g/mol).
Calculate the volume of glacial acetic acid needed using the mass of CH3COOH and the density of glacial acetic acid (1.05 g/mL). Remember that glacial acetic acid is 99% CH3COOH by mass.
Use the Henderson-Hasselbalch equation to find the ratio of [CH3COONa] to [CH3COOH] needed for the buffer: pH = pKa + log([CH3COONa]/[CH3COOH]). Use the given pH (4.50) and the pKa of acetic acid (4.76).
Calculate the moles of CH3COONa needed using the ratio from the Henderson-Hasselbalch equation and the moles of CH3COOH. Then, convert the moles of CH3COONa to grams using its molar mass (82.03 g/mol).
Related Practice
Open Question
Equal quantities of 0.010 M solutions of an acid HA and a base B are mixed. The pH of the resulting solution is 9.2. (a) Write the chemical equation and equilibrium-constant expression for the reaction between HA and B. (b) If Ka for HA is 8.0 × 10⁻⁵, what is the value of the equilibrium constant for the reaction between HA and B?
Textbook Question
Two buffers are prepared by adding an equal number of moles of formic acid (HCOOH) and sodium formate (HCOONa) to enough water to make 1.00 L of solution. Buffer A is prepared using 1.00 mol each of formic acid and sodium formate. Buffer B is prepared by using 0.010 mol of each. (b) Which buffer will have the greater buffer capacity?
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Textbook Question
A sample of 0.2140 g of an unknown monoprotic acid was dissolved in 25.0 mL of water and titrated with 0.0950 M NaOH. The acid required 30.0 mL of base to reach the equivalence point. (a) What is the molar mass of the acid?
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Textbook Question
A sample of 0.1687 g of an unknown monoprotic acid was dissolved in 25.0 mL of water and titrated with 0.1150 M NaOH. The acid required 15.5 mL of base to reach the equivalence point. (b) After 7.25 mL of base had been added in the titration, the pH was found to be 2.85. What is the Ka for the unknown acid?
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Textbook Question
Mathematically prove that the pH at the halfway point of
a titration of a weak acid with a strong base (where the volume
of added base is half of that needed to reach the equivalence
point) is equal to pKa for the acid.
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